North Korea says it has successfully conducted its fifth nuclear bomb test.
State media said the September 9 test had been of a "newly developed nuclear warhead" and that it was now capable of mounting a nuclear device on ballistic rockets.
The announcement came after a huge seismic event was detected near North Korea's Pyunggye-ri nuclear test site, which the U.S. Geological Survey measured as a magnitude-5.3 earthquake.
South Korea says it was Pyongyang's most powerful underground explosion to date.
The test comes after a series of ballistic-missile launches that have drawn international condemnation and United Nations sanctions.
The leaders of South Korea and Japan condemned the latest test as dangerous and a "grave provocation."
South Korean President Park Geun-hye said it showed the "fanatic recklessness of the Kim Jong Un government as it clings to nuclear development."
Japanese Prime Minister Shinzo Abe called the test "absolutely unacceptable" and lodged a protest with Pyongyang.
U.S. President Barack Obama, warning of "serious consequences," called the leaders of South Korea and Japan to consult, White House spokesman Josh Earnest said.
China "firmly opposes" North Korea's nuclear test and "intensely urges" Pyongyang to abide by its nonproliferation promises, said the Foreign Ministry of Pyongyang's only major ally.
Russian Foreign Minister Sergei Lavrov said Moscow was "seriously concerned" by the test, insisting that the UN resolutions "must be strictly implemented."
France "vigorously condemns" the nuclear test and calls on the UN Security Council "to take up this violation of its resolutions," the French presidency said.
South Korea and nuclear experts said the explosion could be almost twice that of the previous nuclear test conducted in January, suggesting a bomb yielding a blast between 10 and 30 kilotons.